Thymic involution is one the major changes that occur in the immune system with age (reviewed in (1)). The reduction of the size and function of the thymus lead to a decreased number of naive T cells in the periphery, compensated by homeostatic proliferation of peripheral memory T cells. As a consequence, the diversity of T cell antigen receptors and the immune responsiveness are reduced in aged individuals. Although the impact of aging on thymocytes, thymic progenitors and thymic microenvironment have been well characterized during the past few years, the genetic determinants and the molecular mechanisms responsible for thymic involution are still poorly understood. We recently discovered that Bcl-Rambo, a member of the Bcl-2 family of apoptotic regulators, is involved in the regulation of thymic involution. Notably, mice lacking this protein display a slower reduction of thymic size with age. The proposed project intends to characterize the role of Bcl-Rambo in the regulation of thymic involution. This study could lead to the identification of potential targets for pharmacological intervention to mitigate thymic involution and its physiological effects. Bcl-Rambo is one of the first proteins described to date, that positively regulates the process of age-related involution of the thymus. With this application we have the opportunity to assess the relative contributions of age-related deficiencies in lymphohematopoietic progenitor function;peripheral or intrathymic signals that regulate involution;and changes in the thymic microenvironment that contribute to the decline in na[unreadable]ve immune cell production, differentiation, and function.